Gros Ventre and Assiniboine Tribes of the Fort Belknap Indian Community Water Rights Settlement Act of 2021 This bill modifies and ratifies a specified water rights settlement agreement entered into by the United States, Montana, and the Fort Belknap Indian Community (i.e., the Gros Ventre and Assiniboine Tribes of the Fort Belknap Reservation of Montana). The bill requires the community's water rights to be held in trust for the benefit of the community and its allottees. The community must enact a tribal water code to regulate its water rights. Among other activities, the Department of the Interior must negotiate with Montana for the transfer of specified parcels of state land located on and off the reservation, hold transferred land in trust for the benefit of the community, allocate 20,000 acre-feet per year of stored water from Lake Elwell, establish a settlement trust fund, and enter into a cost-share agreement with Montana to contribute to the design and construction of the Peoples Creek Dam and Reservoir to support mitigation activities. The bill requires the Bureau of Reclamation to modify the operating procedures of the Milk River Project to ensure that the Fresno Reservoir is operated in accordance with the agreement. The bill also outlines certain waivers, releases, and retention of claims under the settlement agreement.